Since metal halide lamps emit light nearest to natural light, they are excellent in color rendition compared with high pressure sodium lamps or mercury lamps and they are used also as basic illumination in offices or stores.
While, light sources, which are generally used, have high color rendition at an average color rendering index Ra of 80 or more that is higher than the same of color-rendering-section 1B of ISO 8995, and have a color range from so-called warm color at a correlated color temperature in a range from 3,000 to 4,500 K, lamps of higher luminous efficiency have been demanded with a view point of energy saving.
However, high color rendition and high efficiency are effects in a trade off relation, that is, the luminous efficiency lowers as the color rendition is improved, whereas color rendition lowers as the luminous efficiency is improved.
Therefore, while it has been stated that existent metal halide lamps have high efficiency and high color rendition, they are classified either to an efficiency-emphasized type or a color rendition-emphasized type.
In this case, they are generally evaluated to have high color rendition when an average color rendering index is in a range of: Ra≧80 and evaluated to have high efficiency when luminous efficiency is about in a range of: η≧100 (color rendition section according to ISO 8995 of 1B or higher).
For example, since the Dy—Ho—Tm type metal halide lamp disclosed in Patent Document 1 shows the average color rendering index of: Ra=87 and the luminous efficiency of η=93 (1 m/W) the highest data, the lamp can be considered as the color rendition-emphasized type.
Further, since the Na—Ce type metal halide lamp disclosed in Patent Document 2 has excellent average luminous efficiency of: η=123 (1 m/W) due to intense green emission of Ce, but has poor color rendition as the average color rendering index is of: Ra=60 the lamp can be considered as the efficiency-emphasized type (refer to [0049] in Patent Document 2).
Further, while Patent Document 2 describes in [0082] that: “in addition to NaI, dysprosium (Dy), thulium (Tm), holmium (Ho), thallium (Tl), etc. may be added as a luminescent material optionally in accordance with desired lamp characteristics”, even when the ratio of the luminescent material sealed in the Na—Ce type metal halide lamp is controlled by the addition of the materials described above, it is difficult to increase Ra to 70 or more by suppressing the intense green emission of Ce and, in addition, the lamp characteristics described above approaches to those of the Patent Document 1 to lower the luminous efficiency when Dy, Tm, Ho, Ti are added as the luminescent material.